Nobody knows exactly why Apple Computer has invested heavily in the production of industrial

sapphire.

Much of the speculation surrounding the recent deal is centered on the possibility that sapphire could be used — instead of Corning Inc.’s Gorilla Glass — as the screen cover for next-generation iPhones.

Apple’s $578 million investment in a factory in Arizona will allow GT Advanced Technologies to dramatically expand its production of sapphire.

The computer maker already uses small amounts of sapphire in such applications as the lens cover for iPhone cameras.

The unresolved question remains whether high-volume production of sapphire can be achieved that will cut costs enough to make it competitive with Gorilla Glass.

A Gorilla Glass smartphone cover sells for about $3. A similar sapphire cover can run $20 to $30. Sapphire is slightly more scratch-resistant and break-resistant than glass, but not enough to convince phone makers to pay the price difference.

If new production methods can drive down the price of sapphire, it may emerge as a serious rival to Gorilla Glass, which has already been used on 2 billion devices worldwide.

It’s unlikely this will happen overnight.

Gorilla Glass is already in its third generation, with new versions that are lighter, thinner and cheaper likely to emerge in the coming years.

Corning’s fusion draw process, used to produce Gorilla Glass in Harrodsburg, Ky., is a tested manufacturing technique that can be quickly scaled to meet market demand. How soon sapphire makers can match that flexibility is uncertain.

There’s also the question of weight. Sapphire is heavier than Gorilla Glass, and most device manufacturers are looking to cut the weight of their products.

The growth of Gorilla Glass sales has been one of the brightest spots in Corning Inc.’s performance in recent years. A serious slowdown in Gorilla Glass sales would be challenging for the Twin Tiers’ largest employer.

Even if Apple does switch to sapphire, that may have little impact on Corning Inc. Some observers believe Apple has already replaced Gorilla Glass in its iPhones. If that’s the case, a move to sapphire screen covers would have little or no impact on Corning Inc.

Also, Corning is exploring other markets for Gorilla Glass — the automotive and appliance markets, to name just two — that could soften the blow of any rush to use sapphire in smartphones.